Coloring protective coating on magnesium and its alloys



atented Feb. 9, 1943 @UWRWG PRQTWE COATING ON MAGNES H'ES ALLOYS HerbertKDe Long, Mind, Mich, assignor to The Dow Chemical irchigan acorporation of my, Midland, Mich,

No Drawing. Application January 17, 1941, Serial No. 374,893

This invention relates to an improved method of coloring hydrated oxideprotective coatings upon the surface of articles of magnesium andmagnesium-base alloys.

Articles of magnesium and magnesium-base alloys are commonly providedwith some form of chemical finish to prevent attack of the metal bycorrosive agents to which it may be exposed. One of the known finishes,the one to which the present invention is directed, consists essentiallyof an adherent coating of hydrated oxide formed on the surface of themagnesium article by interaction of the base metal with water, usuallyin the presence of alkalies. Such coatings may be produced as thickrelatively sot-t layers by sub- ,iecting the article to the action ofhot water at elevated pressures, (as described in U. S. Patent Reissue16,340) or by immersing it in a boiling alkaline solution of anoxidizing agent, (as disclosed in U. S. Patent 1,642,309). Moredesirably, the oxide coatings may be prepared in hard,

'2 has. (Cl; 148-45) magnesium-base alloys are colored by immersing thecoated articlein an aqueous solution of a water soluble dye and a watersoluble salt of an unsubstituted organic carboxylic acid. In this wayuniform colorings of any desired shade may easily be prepared.

Among the salts which may be employed in the dyeing solution, the alkalimetal, ammonium, and aluminum salts of oxalic acid and such carboxylicacids as the saturated aliphatic acids, e. g. formic, acetic, andpropionic acids, alicyciic acids,'e. g. cyclohexane carboxylic acidjaromatic acids, e. g. benzoic, and aralkyl carboxylic acids, e. a.phenyl acetic acid, are most suitable. Salts of acetic acid, especiallyammonium acetate, aluminum acetate, and th alkali metal acetates, are

' tained with these agents are unusually deep and dense, thin form,either by subjecting the magnesium article to the action of a 2 to percent water solution of alkali metal hydroxide under pressure attemperatures of 150 to 250 "C., (as set forth in the publicationDowmetal Bulletin DM3a, and in my co-pending applications, Serial No.224,510, filed August 12, 1938, now U, S.

Patent No. 2,250,472, and Serial No. 348,681, filed July 31, 1940), orby anodizing the article in an aqueous polyhydric alcohol solution of analkali at temperatures of 50 to 150 C. (as explained in detail in myco-pending application Serial No. 369,628, filed December 11, 1940).

As is known, hydrated oxide protective coat= lugs upon magnesiumarticles, such as those described, may be colored to produce decorativeeflects either by incorporating a soluble dye in the solution used informing the coating, or, in some instances, by dipping the coatedarticle in a water solution of the dye. Unfortunately the former methodis disadvantageous in that, although satisfactory colorings areobtained, the dye is gradually decomposed at the operating temperaturesand must be replaced. The latter methoddoes not always give uniformcolorlngs, is limited in the number of colors which can be thus applied,and is not applicable to the hard,

true, and the process is very easily controlled. In general, the saltshould be present in a concentration between about 0.1 and about 10grams per liter of solution, preferably 0.2 to 2 although otherconcentrations are operable.

Salts of substituted carboxylic acids, such as the I hydroxy-allphaticacids, e. g., citric and tartaric acid, are not satisfactory.

A variety of soluble dyes may be used in-the invention, However, it hasbeen found that the Neolan dyes, i. e. chromium-containing mordant azodyes, particularly organo-chromium'complex mordant ortho-hydroxy azodyes (asdescribed in Melliands Textilber. 6, 6746 (1925), or in J. Soc.Dyers and Colourists 50, 282 (1934)), are especially suitable. Typicaldyes which have been used include:

Neolan Green B Neolan Green BL (conc.) 'NeolanBo'rdeaux R (redY NeolanOrange G Neolan Yellow G Neolan Black 2R Alizarin Sapphire Blue (ColourIndex No.

1054) 1 Naphthol Yellow S (Colour Index No. 10)

required to produce an excellent coloring. The dyeing treatment has noappreciable effect n the corrosion resistance of the hydrated oxidecoatings.

In forming a colored article, the magnesium or magnesium-base alloyarticle is first subjected to a suitable treatment, such as thosealready considered, to form a hydrated oxide protective coating. Thearticle is then buifed or brushed, if necessary, to remove any loosematerial and to produce a high gloss, after which it is immersed in thedye solution to color the coating. The article is then rinsed and dried,after which it may, if1 desired. be waxed or polished to improve the goss.

The following example will illustrate the invention: 1

Example Articles of extruded Dowmetal J alloy (an alloy containing 6.5per cent aluminum, 0.2 per cent manganese, and 0.7 per cent zinc, thebalance being magnesium) were treated to form a hard, dense, thinhydrated oxide protective coating by immersing them for 5 minutes in aper cent aqueous sodium hydroxide solution maintained at a temperatureof 200 C. under a pressure of 225 lbs. per sq. in. The articles werethen rinsed and dyed by immersing them for 10 minutes in a boilingaqueous solution of 0.2 per cent ammonium acetate and 1.0 per cent ofNeolan Bordeaux R.

The articles were dyed a pleasing uniform red scription is illustrativerather than strictly limitative of the invention.

I claim:

1. In a method of coloring hydrated oxide protective coatings uponsurfaces of articles of magnesium and magnesium-base alloys, the stepwhich comprises immersing the coated article in an aqueous solution of awater-soluble dye and a water-soluble salt of an unsubstituted organiccarboxylic acid.

2. A method as defined in claim 1 wherein the water-soluble salt is asalt of acetic acid.

3. In a method of coloring hydrated oxide protective coatings uponsurfaces of articles of magnesium and magnesium-base alloys, the stepwhich comprises immersing the coated article in a substantially neutralaqueous solution of a water-soluble dye and a water-soluble salt of anunsubstituted organic carboxylic acid.

4. In a method of coloring hydrated oxide protective coatings uponsurfaces of articles of magnesium and magnesium-base alloys, the stepswhich comprise immersing the coated article in an aqueous solution of achromium-containin mordant azo dye and a water-soluble salt of anunsubstituted organic carboxylic acid.

5. A method as defined in claim 4 wherein the water-soluble salt is asalt of an acid selected from the class consisting of saturatedaliphatic acids, alicyclic acids, aromatic acids, and aralkyl carboxylicacids.

6. A method as defined in claim 4 wherein the water-soluble salt is asalt of an unsubstituted aliphatic carboxylic acid.

7. A method as defined in claim 4 wherein the water-soluble salt is asalt of acetic acid.

8. In a method of coloring hydrated oxide protective coatings uponsurfaces of articles of magnesium and magnesium-base alloys, the stepwhich comprises immersing the coated article in a substantially neutralaqueous solution containing a water-soluble dye in a proportion betweenabout 0.2 and about 10 grams per liter and a water-soluble salt of anunsubstituted organic carboxylic acid in a proportion between about 0.1and about 10 grams per liter.

9. In a method of coloring hydrated oxide protective coatings uponsurfaces of articles of magnesium and magnesium-base alloys, the stepwhich comprises immersing the coated article in an aqueous solutioncontaining a chromiumcontaining mordant azo dye in a proportion betweenabout 0.2 and about 10 grams per liter and a water-soluble salt of anunsubstituted aliphatic carboxylic acid in a proportion between about0.1 and about 10 grams per liter.

10. A method as defined in claim 9 in which the water-soluble salt is asalt of acetic acid.

11. In a method of coloring hard, dense, thin hydrated oxide protectivecoatings upon surfaces of articles of magnesium and magnesium-basealloys, the step which comprises immersing the coated article for aperiod between about 5 and about 30 minutes in a solution containing awatersoluble ortho-hyoroxy azo dye in a proportion between about 0.2 andabout 10 grams per liter and a water-soluble salt of acetic acid in aproportion between about 0.1 and about 10 grams per liter, the solutionbeing maintained at a temperature between about 50 C. and about C.

12. A method as defined in claim 11 wherein the acetic acid salt isaluminum acetate.

13. A method as defined in claim 11 wherein the acetic acid salt isammonium acetate.

14. A method as defined in claim 11 wherein I the acetic acid salt is analkali metal acetate.

15. Articles of magnesium and magnesium-base alloys having on thesurface thereof a hydrated oxide protective coating colored according tothe method of claim 1.

16. Articles of magnesium and magnesium-base alloys having on thesurface thereof a hydrated oxide protective coating colored according tothe method of claim 4.

17. Articles of magnesium and magnesium-base alloys having on thesurface thereof a hydrated oxide protective coating colored according tothe method of claim 9.

18. Articles of magnesium and magnesium-base alloys having on thesurface thereof a hydrated oxide protective coating colored according tothe method of claim 11.

HERBERT K. DE LONG.

organo-chromium complex mordant,

